Many problems can be pinpointed on custom content or corruption in your user data. There's an easy way to reset your game to factory settings without going through the ordeal of reinstalling the whole thing (Path can be different depending on your OS):

Make a backup of theSims4folder in Documents\ElectronicArts to an external drive or a different place on your harddrive (or burn it on a DVD or to a flash drive). This is only a backup if anything goes wrong. You won't need this folder for the further steps.

Out-dated graphic and sound drivers, missing Windows Service Packs and other components that are not up-to-date can cause problems with your game. Make sure you you update those regularly.

Computer maintenance is important. Defrag your Computer, clean your Registry and get rid of temporary files on a regular basis. Also clean the ventilators from time to time and if you have a desktop open it up and clean it inside.

Backup your important files. That goes for your music, your photos as well as your Sims User Files. You'll find the user files in Documents\ElectronicArts. Copy the whole folder to an external drive.

They are both very important and no Computer should be without. Nevertheless can they lead to issues. If you have trouble patching or the launcher doesn't work correctly, make sure you tell your Virus program/Firewall that the EA Site is safe. Disable it temporarily to find out if the firewall is the issue.

I don't work or have any association with EA. I give advice to the best of my knowledge and cannot be held responsible for any damage done to your computer/game.Questions belong in the forums. I don't answer questions via PM.Help us helpers to help youThinking of helping out ? Please check the Helper's Corner for Sims and The Academy

You only need to activate the game once. After that it's registered to the account you've activated your game and that cannot be changed.To reinstall your game, follow these steps:
If you don't have the Origin client installed on your computer yet, you can download it here: http://origin.com/download
Log in with the account you have registered the game on (use your email address and not user username
Find the game in myGames to download/install
If the game is no longer shows up on your Origin client, please try the following things
Restart Origin
Log out and back in and use the email address instead of the username. Make sure to use the one you used to register the game in the first place
Reinstall Origin
Restart your router/modem,
Get in touch with a game advisor.

Most graphical issues are can either be pinpointed to graphics chips which are below specs or outdated drivers. Because of this, you should check if you fulfill the requirements of the game and that you have the newest drivers.

Play around with your graphics settings. Especially with the following options: Post Processing Effects, Laptop Mode

Most graphical issues are can either be pinpointed to graphics chips which are below specs or outdated drivers. Because of this, you should check if you fulfill the requirements of the game and that you have the newest drivers.
Lists to check if your card is good enough: Game Help:TS4 System RequirementsSupported Graphics Card List (The Sims 4 Edition)
Updateing your drivers: AMD, Nvidia,Intel
If you have a dual graphics card system with an Chip/Card combo, make sure that the game runs on the card.
Play around with your graphics settings. Especially with the following options: Post Processing Effects, Laptop Mode

The game never worked

Make sure your antivirus software/firewall are not blocking the connection. It's not always enough to create an exception especially if the program has already blicked the game. Check your firewall/antivirus software's manaul on how to proceed. Comodo Firewall seems to be notorious for blocking. Add the following programs to your exception list: authorizationUI, TS4.exe, Origin.exe

The game suddenly doesn't start anymore

First thing to try when your game doesn't start: Remove your Mods. (Why ?)
The game never worked
Start Origin and the game as administrator
Use ccleaner to clean your registry.
Delete the Origin cache files.
Check your hardware, esp. chkdsk, to make sure your hard drive doesn't have any issues
Make sure your antivirus software/firewall are not blocking the connection. It's not always enough to create an exception especially if the program has already blicked the game. Check your firewall/antivirus software's manaul on how to proceed. Comodo Firewall seems to be notorious for blocking. Add the following programs to your exception list: authorizationUI, TS4.exe, Origin.exe
Start the game in windowed mode. If the game runs, lower all settings and put it back to full-screen mode.
Eliminate issues with Origin
Deactivate Origin ingame
Reinstall Origin and the game and make sure they are on the same drive, preferably c:
Repair the game through Origin
Clean Boot your PC
Play in offline mode
Make sure that you don't have programs that have a known conflict with Origin
Update your graphics drivers.
VPNs can lead to potential conflicts with the game
Make sure your computer fulfills the requirements of the game.
Make sure your Windows is updated
Make sure the game is updated (Right-click on the Game in Origin and choose Search for Updates).
Network Troubleshooting:
How can I reset the Hosts file back to the default?
How to flush DNS Cache for Windows
The game suddenly doesn't start anymore
On top of the things above you can also try this
Reset your user files
Deactivate newly installed programs

Good Luck and Happy GamingCrin

I don't work or have any association with EA. I give advice to the best of my knowledge and cannot be held responsible for any damage done to your computer/game.Questions belong in the forums. I don't answer questions via PM.Help us helpers to help youThinking of helping out ? Please check the Helper's Corner for Sims and The Academy